The long-range objectives of this project are to develop practicable procedures for classification of spotted fever- and typhus-group rickettsiae and to determine the nature and biological properties of rickettsial antigens and constituents. This part of the project focuses on differentiating serologically the kinds of rickettsiae found in arthropod vectors known to transmit rickettsial disease agents to man or are involved in the maintenance of pathogenic rickettsiae in nature with particular reference to the United States. The microimmunofluorescence (micro-IF) test is an excellent screening test for this purpose. Ultimately, representative strains of the important serotypes will be systematically examined by more refined techniques for antigenic and other biologic characteristics.